Cumulative patch with refactored Raniz's changes

Support for package signing for maven plugin, including:

* Automatic lookup of secure ring location (keyring) if not set.

* Support for reading signing properties (keyring, key and
  passphrase) from settings, allowing storage of sensitive information
  outside the pom.

* Configurable prefix to define different keys for different packages,
  the default prefix is "jdeb." resulting in the properties
  jdeb.keyring, jdeb.key and jdeb.passphrase being read from the
  properties in the active profiles.

* Log message stating that we're signing the `.deb`.

* Documentation for signing packages.